Author(s)

Bruce Hoeneisen

Universidad San Francisco de Quito, Quito, Ecuador.

Collisionless dark matter can only expand adiabatically. To test this idea and constrain the properties of dark matter, we study spiral galaxies in the “Spitzer Photometry and Accurate Rotation Curves” (SPARC) sample. Fitting the rotation curves, we obtain the root-mean-square (rms) velocity and density of dark matter in the core of the galaxies. We then calculate the rms velocity v_hrms (1) that dark matter particles would have if expanded adiabatically from the core of the galaxies to the present mean density of dark matter in the universe. We obtain this “adiabatic invariant” v_hrms (1) for 40 spiral galaxies. The distribution of v_hrms (1) has a mean 0.87 km/s and a standard deviation of 0.27 km/s. This low relative dispersion is noteworthy given the wide range of the properties of these galaxies. The adiabatic invariant v_hrms (1) may, therefore, have a cosmological origin. In this case, the rms velocity of non-relativistic dark matter particles in the early universe when density perturbations are still linear is v_hrms (a)=v_hrms (1)/a, where a is the expansion parameter. The adiabatic invariant obtains the ratio of dark matter temperature T_h (a) to mass m_h in the early universe.

Spiral Galaxies, Dark Matter, Dark Matter Properties

Hoeneisen, B. (2019) The Adiabatic Invariant of Dark Matter in Spiral Galaxies. International Journal of Astronomy and Astrophysics, 9, 355-367. doi: 10.4236/ijaa.2019.94025.